This invention relates to an air/fuel ratio control system for controlling the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine, and more particularly to an acceleration control device provided in such control system, for performing air/fuel ratio control in a predetermined manner when the engine is accelerated from its idle state.
An air/fuel ratio control system has already been proposed, e.g., by the assignee of the present application, which is arranged to perform feedback control of the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine having an exhaust system provided with a three-way catalyst, which comprises means for detecting the concentration of an ingredient in the engine exhaust gases, fuel quantity adjusting means for producing the mixture being supplied to the engine, and means operatively connecting the concentration detecting means with the fuel quantity adjusting means in a manner effecting feedback control operation in response to an output signal produced by the concentration detecting means to control the air/fuel ratio of the mixture to a preset value, the connecting means including a valve for varying the air/fuel ratio of the mixture being supplied to the engine, a pulse motor for driving the valve, and an electrical circuit arranged to drive the pulse motor in response to the output signal of the concentration detecting means. The air/fuel ratio control system is thus capable of achieving improved engine driveability as well as improved engine exhaust gas emission characteristics.
The operating states of an internal combustion engine where detrimental gases are contained in large quantities in the exhaust gases under normal operating conditions of a vehicle on which the engine is installed, include the so-called "standing start" which means starting the vehicle from its standing position. That is, when the accelerator pedal of the vehicle is stepped on to accelerate the engine from its idle state, the mixture being supplied to the engine becomes too rich due to the action of an acceleration pump mounted on the engine. This causes an increase in the amount of unburnt ingredients in the exhaust gases. Further, on this occasion, the suction air amount increases due to wide opening of the throttle valve to increase the charging efficiency of the engine so that the combustion temperature rises, which results in an increase in the amount of NOx present in the exhaust gases.
According to the aforementioned proposed air/fuel ratio control system, there is a response lag between the time of the standing start and the time of the engine shifting into a normal operating condition if the air/fuel ratio feedback control is conducted on the basis of detection of the concentration of an engine exhaust gas ingredient, which makes it impossible to achieve accurate air/fuel ratio in quick response to sudden acceleration at the standing start of the vehicle. As a result, a required air/fuel ratio cannot be achieved at the start of normal operation of the engine under air/fuel ratio feedback control immediately after the standing start of the vehicle, thus deteriorating the exhaust gas emission characteristics of the engine. More specifically, in addition to emission of a large amount of detrimental gas ingredients at the standing start of the vehicle, there is also the occurrence of detrimental gas ingredients in large quantities at normal operation of the engine immediately after the standing start, which leads to a great increase in the total amount of detrimental gas ingredients or pollutants in the exhaust gases from the standing start to the subsequent normal operation.